Access networks are networks that are costly to the telecommunications operators because they are more often than not tree-structured networks serving numerous subscribers. Such networks are equipped with numerous components that often consume electrical energy. In order to limit these operating costs while improving the quality of the services offered to the subscribers, the operators have developed passive optical access networks. All the components located in the network between the optical exchange and the user equipment are passive, that is to say that they do not need to be electrically powered to function.
Such optical networks offer subscribers a high connection bit rate of the order of 2.5 Gbit/s (Gigabits per second). These bit rates make it possible to offer services such as high definition television, Internet or even videophony, so meeting a demand from the subscribers.
They are currently implemented for the deployment of high bit rate access to the homes of residential subscribers of FTTH (Fiber To The Home) type.
A passive optical network of bidirectional PON type is known from the document entitled “Bidirectional WDM/TDM-PON access Networks integrating downstream 10 Gbits/s DPSK and upstream 2.5 Gbits/s OOK on the same wavelength”, by Genay et al, published as paper Th3.6.6 at the European Conference on Optical Communications (ECOC) 2006, held in Cannes, France. Such an optical access network 1 comprises, with reference to FIG. 1, an optical exchange 10 linked by a bidirectional optical fiber 20 to a 1-to-N distribution element 30, N being an integer greater than or equal to 1, able to distribute a downlink optical signal to N line termination devices 501 to 50N and to multiplex N uplink optical signals transmitted by the N line termination devices 501 to 50N to the optical exchange 10. The distribution element is linked to the line termination devices 501 to 50N by N optical fibers 401 to 40N. To each line termination device one or more subscribers can be connected.
The optical exchange 10 comprises means 11 of transmitting an optical signal, generally a laser used to convey information addressed to one or more subscribers, means 12 of receiving an uplink optical signal originating from the subscribers and a circulator 13, able to make the downlink and uplink optical signals circulate in the single optical fiber 20.
The line termination device 50i comprises a circulator 51i able to make the downlink SOD,ri and uplink SOR optical signals circulate in the bidirectional optical fiber 40i. It also comprises coupling means 52i able to distribute the optical power of the received downlink optical signal SOD,ri between a first SOD,ri1 and a second SOD,ri2 received downlink optical signal. The first received downlink optical signal SOD,ri1 is processed by reception means 53i for decoding. The second received downlink optical signal SOD,ri2 is processed by means 54i of generating an uplink optical signal SORi from the received downlink optical signal SOD,ri.
The passive optical network described previously uses the principle of time-division multiplexing or TDM. In such a network, the optical signal transmitted by the laser 10 is divided up into a plurality of time slots of the same duration. Each time slot is then associated with one of the line termination devices 50i according to their requirements.
There are also passive optical networks that use wavelength division multiplexing or WDM. In such a network, the optical exchange comprises a plurality of lasers each transmitting an optical data component associated with a wavelength that is specific to them. An optical multiplexer placed at the output of the optical exchange and to which is connected a first end of the main fiber of the network is used to inject into the latter a wavelength-division multiplexed signal. In such an optical network, each line termination device is associated with an optical component obtained from the optical exchange and therefore with a particular wavelength.
The passive optical access networks, whether they use time-division multiplexing or wavelength-division multiplexing, offer a conventional range of the order of 20 km (kilometers). This limited range of the network results from the fact that, in the passive optical networks, the various optical components that are, for example, the optical couplers, the optical multiplexers or the optical fibers, bring about optical power losses in the signals passing through the network and that the transmitted signals cannot be amplified without constraints to compensate such losses. In practice, in a passive optical network, the downlink optical signals, that is to say the optical signals transmitted by the exchange to the subscribers, and the uplink optical signals, that is to say the optical signals transmitted by the subscriber equipments to the optical exchange, are conveyed by a single optical fiber. This reduces the cost of the network. However, the use of a single optical fiber to convey the downlink and uplink optical signals introduces constraints on the transmission power levels of these optical signals.
Notably, it is essential on the one hand for the transmission power of the data signals to be sufficient to compensate the losses associated with crossing the network and thus allow for correct reception. It is also essential for the power not to be high to the point of generating backscattered signals that could dazzle the reception means used to detect the signals being propagated in the subscriber-exchange direction. The result of this trade-off on the value of the transmit optical power of the signals in a passive optical network is reflected in a limited network range.
One current trend is to increase the transmission bit rate in the passive optical access networks beyond 10 Gbit/s, to 40 Gbit/s. Such an increase in bit rate leads to an increase in the distortions that are undergone by the signals transmitted through the optical access network. These distortions, that are emphasized with the bit rate, include the phenomenon of chromatic dispersion (CD). Given a constant range (therefore given constant aggregate chromatic dispersion), the impact of the chromatic dispersion is multiplied by 16 for a bit rate multiplied by 4.
The solutions of the prior art consist in introducing modules for compensating the in-line chromatic dispersion. A first drawback of such devices is that they are costly. A second drawback is that they are therefore unsuited to an access network of PON type, because the line termination devices are not necessarily all located at the same distance from the optical exchange (it would require a compensation specific to each customer for everything to be perfect).
Thus, there is a need to compensate the chromatic dispersion introduced upon the transmission of optical signals over a high-bit rate passive optical network, typically 10 Gbit/s and beyond, while maintaining the passive nature of such a network.